This invention relates to apparatus and methods of countergravity casting of molten metal. More particularly, the invention concerns such casting in which the molten metal is caused to flow into and fill the cavities of gas-pervious molds by low pressure induced in a vacuum chamber sealed around them.
Prior art apparatus and methods of the type concerned set forth in U.S. Pat. Nos. 3,900,064 and 4,589,466 have been successful in producing high quality castings, superior in many respects to castings produced by pouring methods dependant on gravity-induced flow. The vacuum chamber is usually maintained at a pressure at least as low as about 1/3 (5 p.s.i.) below atmospheric pressure while the molten metal is essentially at atmospheric pressure and, to fill thin molding cavities, often as low as 13 p.s.i. below atmospheric pressure.
Also, after the mold is filled, the metallostatic pressure in the lower part of the mold is additive to the vacuum pressure, so the total metal pressure in that volume often reaches 18 psi. These metal pressures generate stresses in the mold walls depending on the shape of the mold cavity and its size. The size of these stresses increases as the parts overall dimensions increase. For example, a part 2".times.4".times.1/4" could have a force of 144 lbs. to contain while a part 6".times.4".times.1/4" would have a force of 432 lbs. to contain. Such high forces when combined with the high temperatures of steels especially, can cause mold wall movement, metal penetration into the mold face, and even outright mold failure especially if there are any structural defects in the molds. The practical effect is that costly measures may be required to avoid these problems and certain larger shapes cannot be made by the methods taught. Also, the methods require molds of high strength and inside faces of low porosity, such as high temperature bonded ceramic shell molds. Lower strength molds, such as low temperature bonded sand molds, have been filled primarily by other methods, such as the partial immersion of the mold in the molten metal with vacuum applied only to the upper part of the mold, in accordance with U.S. Pat. Nos. 4,340,108 and 4,532,976.